Polymers are used extensively in oil field application as fluid additives for drilling, cementing, gas and oil well fracturing and enhanced-oil-recovery processes. Synthetic, organic, and inorganic polymers, as well as cellulose ethers and guar gum and guar derivatives, are widely used in oil field applications. These materials are also applied in a variety of formation-damage control applications and as dispersing agents.
In the initial drilling operation of an oil or gas well, a drilling fluid, commonly referred as “drilling mud,” is pumped under pressure down to a string of drill pipes through the center of the drilling bit, back through the space or annulus between the outside of the drilling stem and the borehole wall, and finally back to the surface. After a well has been drilled and oil discovered, one or more subterranean, hydrocarbon-producing formations are most often encountered. The well is then completed to obtain the maximum hydrocarbon production from the subterranean producing formations.
Completion of a well refers to the operations performed during the period from drilling-in the pay zone until the time the well is put into production. These operations may include additional drilling-in, placement of downhole hardware, perforation, sand control operations, such as gravel packing, and cleaning out downhole debris. A completion fluid is often defined as a wellbore fluid used to facilitate such operations. The completion fluid's primary function is to control the pressure of the formation fluid by virtue of its specific gravity. The type of operation performed, the bottom hole conditions, and the nature of the formation will dictate other properties, such as viscosity. Use of completion fluids also clean out the drilled borehole. Oil well cement compositions are used in the completion operation to make a permanent, leak proof well for continuous use.
In cementing operations of gas or oil wells, hydraulic cement is normally mixed with sufficient water to form a pumpable slurry and the slurry is injected into a subterranean zone to be cemented. After placement in the zone, the cement slurry sets into a hard mass. In primary cementing, where cement slurry is placed in the annulus between a casing or liner and the adjacent earth formations, loss of fluid is a major concern. The formations can result in premature gelation of the cement slurry and bridging of the annulus before proper placement of the slurry. In remedial cementing operations, the control of fluid loss is necessary to achieve the more precise cement slurry placement associated with such operations.
Among all other slurry properties, fluid loss control is one of the critical concerns for cement slurry formulation, especially at high temperature, high pressure (squeeze cement) and salt environments. The main purpose of fluid loss additives is to prevent the dehydration of the cement slurry that can reduce its pumpability as well as affecting its other designed properties. Loss of a significant amount of water from the cement slurry can cause changes in several important job parameters, such as reduced pumping time and increased frictional pressure. Fluid loss additives are used to help prevent water loss from cement slurries to the rock formation as the slurry is pumped into the annulus between the casing and the well bore. This allows displacing the maximum amount of mud, compressive strength development, and bonding between the formation and the casing. In fact, under harsh conditions and due to permeable zones, the slurry can dehydrate quickly and become unpumpable, preventing the extension of slurry into voids and channels, particularly where the annular space between the liner and the open hole is too narrow. Any bridging problem due to high fluid loss would considerably disturb the cement job and affect the integrity of the cement column.
A large variety of synthetic polymeric fluid loss additives have been used in drilling fluid and oil-well cement slurries. These synthetic polymeric fluid loss additives are mainly used for high temperature operations. Among many other mechanisms of action, it is well known that the fluid loss control efficiency is greatly affected by the molecular weight of the polymeric additive. However, for cement slurries, high Mw polymers (>1,000,000 Daltons) exhibit an undesirable side effect related to a viscosity increase of the slurry. Low Mw versions (<1,000,000 Daltons) are useful to prevent the viscosity side effect, but the slurry stability could be affected by cement settling and free water due to lack of suspending capacity of those polymers. It has been found that by using a selected hydroxyethyl cellulose in combination with a low Mw synthetic polymer, a pumpable liquid product that is effective in fluid loss control without a free water problem can be obtained.
Previously, it was not possible to prepare a liquid mixture of these two polymers that has a viscosity low enough to be pumpable. In fact, hydroxyethyl cellulose has a tendency to dissolve in the mixture, resulting in a gel like solution.
U.S. Pat. No. 4,883,536 describes suspensions of hydroxyethylcellulose prepared in concentrated aqueous solutions of diammonium sulfate or diammonium phosphate, and that these suspensions in turn were useful for rheology modification of latex paint. U.S. Pat. No. 5,228,908 describes that concentrated aqueous sodium formate solution can be employed for preparation of fluid suspensions of the polymers such as hydroxyethylcellulose as well. In U.S. Pat. No. 5,578,168, the use of a number of salts, including sodium formate, were shown to be useful for preparing aqueous suspensions of poly(ethylene oxide). In U.S. Pat. Nos. 5,541,241 and 5,489,638, it was demonstrated that sodium formate and other salts were useful for preparing aqueous suspensions of polyvinyl alcohol.